Stardock does indeed make such software. It's called DirectSkin, and it uses the same skin format as WindowBlinds. In a nut shell, the software developer includes the DirectSkin OCX in their application, which makes it skinnable. The developer can choose whether they want to allow the user to select skins, or if they just want to use a single skin. As far as I know, it does not require the developer to re-write the software. They simply need to recompile it with the DirectSkin OCX. You can learn more about it here:
http://www.directskin.com